In recent years, a surround system is being practically used, in which each of a plurality of speakers such as a center speaker, right and left front speakers or right and left rear speakers (also called surround speakers) has the role of reproduction sounds, and addition of reverberant sound, change in the frequency characteristic, and the like is performed in each of the speakers, thereby amplifying sound such as voice and music.
A known representative surround system is a Dolby™ digital 5.1ch (channel) surround system constructed by a center speaker and front speakers installed on the right and left sides of the center speaker, which are provided forward of a listener, surround speakers installed on the right and left rear sides or right and left sides of the listener, and a sound woofer dedicated to amplify sound only in the lower audio frequencies of 120 Hz or less. There is also a known conventional method for use in such a 5.1ch surround system, in which right and left surround speakers are formed integrally and installed rearward of a listening position to amplify sound so that the surround speakers can be set easily.